1. Field of the Invention
The present invention relates to lithography, and more particularly, to an active faceted mirror system.
2. Background of Invention
Lithography is often used in the manufacture of many devices and in particular, electronic and semiconductor devices, flat panel displays, print heads, micro/nano fluidic devices and the like. In a lithographic process, an image is projected onto a photosensitive substrate. As the element or feature size (hereinafter referred to as “feature size”) desired to be imaged on the photosensitive substrate becomes smaller, technical problems often arise. One of these problems is providing illumination, so that its image can be projected onto the photosensitive substrate. As the feature sizes of semiconductor devices becomes smaller, there is a need for photolithographic systems that provide a resolution of less than 0.065 micrometers. In order to achieve the imaging of these relatively small element or feature sizes, shorter wavelengths of electromagnetic radiation must be used to project the image onto a photosensitive substrate. Accordingly, it is often necessary for lithographic systems to operate at the extreme ultraviolet (EUV) wavelengths, below 157 nanometers, and into the soft x-ray wavelengths, around 1 nanometer.
Historically, there were few illumination systems that could provide the required illumination properties for projecting the image of the reticle or mask onto a photosensitive substrate at these operating wavelengths. An illuminating system is disclosed in U.S. Pat. No. 5,339,346 entitled “Device Fabrication Entailing Plasma-Derived X-Ray Delineation” issuing to White on Aug. 16, 1994, which is herein incorporated by reference in its entirety. Therein disclosed is a condenser for use with a laser-pumped plasma source having a faceted collector lens including paired facets, symmetrically placed about an axis.
Another illumination system is disclosed in U.S. Pat. No. 5,677,939 entitled “Illuminating Apparatus” issuing to Oshino on Oct. 14, 1997, which is incorporated herein its entirety. Therein disclosed is an illumination system for illuminating an object in an arcuate pattern. The system has a reflecting mirror with a parabolic-toric body of rotation and a reflection type optical integrator having a reflecting surface for effecting the critical illumination in the meridoinal direction and a reflecting surface for effecting the Kohler illumination in the sagittal direction.
Another illumination system is disclosed in U.S. Pat. No. 5,512,759 entitled “Condenser for Illuminating A Ring Field Camera with Synchrotron Emission Light” issuing to Sweatt on Apr. 30, 1996, which is herein incorporated by reference in its entirety. Therein disclosed is a condenser comprising concave and convex spherical mirrors that collect the light beams, flat mirrors that converge and direct the light beams into a real entrance pupil of a camera, and a spherical mirror for imaging the real entrance pupil through the resistive mask and into the virtual entrance pupil of the camera.
Another illumination system is disclosed in U.S. Pat. No. 5,631,721 entitled “Hybrid Illumination System for Use in Photolithography” issuing to Stanton et al on May 20, 1997, which is herein incorporated by reference in its entirely. Therein disclosed is a multi-stage optical element, a condenser, and an array or diffractive optical element.
In some circumstances, these prior illumination systems may not provide the desired illumination and are relatively complicated. Additionally, many of these systems are relatively large, having many surfaces resulting in loss of energy. Some are also difficult to align and may require adjustment.
Another illumination system using a reflective fly's eye condenser is disclosed in U.S. Pat. No. 6,195,201 entitled “Reflective Fly's Eye Condensor for EUV Lithography” issuing to Koch et al. on Feb. 27, 2001, which is incorporated herein in its entirety. The illumination system disclosed in this patent addressed some of the shortcomings of the prior systems by providing an improved illumination system and condenser for use in the extreme ultraviolet that provides a desired irradiance over a predetermined field or area with a desired irradiance and angular distribution, pupil fill, or radiant intensity for use in photolithography. U.S. Pat. No. 6,195,201 achieved some of these improvements over existing illumination systems through the use of reflective fly's eyes, which are also known as a faceted mirrors or mirror arrays within an illumination system.
These faceted mirrors are referred to as fly's eyes because they consist of a set of many small mirrors, which can be referred to as chiclets, precisely configured on a base to achieve a desired reflection. An illumination system or condenser for use within a photolithographic system can consist of an illumination source that irradiates a first faceted mirror that reflects electromagnetic energy to a second faceted mirror. The first faceted mirror is typically referred to as field faceted mirror and the second faceted mirror is typically referred to as a pupil faceted mirror. Electromagnetic energy reflected from the pupil faceted mirror can typically be reflected through a series of optical elements to form an illumination field on a reticle or mask.
While the use of fly's eye mirrors has the potential to provide a precise mechanism to direct electromagnetic energy in an EUV photolithography system, current illumination systems using fly's eye mirror have significant shortcomings. Illumination systems using fly's eye mirrors that are contemplated typically would be contained within very expensive lithography systems. Customers that would use these lithography systems need to manufacture semiconductor devices that have a wide range of characteristics, and as such require a lithography tool that can support a wide range of illumination needs. Current illumination systems using fly's eye mirrors that are contemplated can not effectively support these wide range of needs. Because of the size and complexity of fly's eye mirrors, it is difficult to change out or adjust the mirrors within a given system to meet different manufacturing needs. Similarly, lithography systems that would have different combinations of fly's eye mirrors are expensive and can be cost prohibitive.
What is needed is an active faceted mirror system for use within a lithographic system that can cost effectively be adjusted to meet varying photolithographic manufacturing demands.